If you stack 100 footcandles on top of 100 footcandles and you get anything but 200 footcandles, where do the missing footcandles go? …There’s a lot more going on than meets the eye (or the meter). Since LEDs are driven by pulses of current, they are constantly flashing on and off at a very high rate. Depending on the sample window of the illuminance meter, the timing of pulses, and the pulse width, the results of the meter reading will vary. If, for example, the pulses are completely out of sync with each other and the “on” times never overlap, it’s possible that the illuminance would never exceed that of a single fixture. If, on the other hand, the pulses were completely synched, then the meter could read double the illuminance of a single fixture depending on the sampling period of the meter. The odds are that the LEDs will be somewhat out of sync but the pulses would overlap to a degree, leading to a reading somewhere between that of a single fixture and twice that of a single fixture. The same holds true of discharge lamps that are powered by magnetic ballast power supplies. (Electronic power supplies have a higher frequency than the meter sampling rate and the meter is more likely to read more accurately.)
—Richard Cadena, from Focus on Fundamentals, PLSN, April 2009
